Introduction

As the rate of security systems have been steadily increasing, so has the need for innovations for door and window contact sensors. Door and window sensors are often used in home security on each door and window throughout a household as a foundation to any home security system. Contact sensors are an essential part of security systems as the sensors alert a system to the opening or closing of each monitored entry point. Historically, reed switches were the design used in contact sensors due to their simplicity; however, reed switches provide erratic detection fields which leaves the system vulnerable to tampering unlike Hall-effect sensors.

If a door or window is opened while the alarm system is active, the main alarm can be immediately triggered when the Hall-effect sensor sends an alert to the main control panel. Contact sensors are mostly battery powered where the Hall-effect sensor alternates between active measure mode and low power sleep mode to reduce the average current consumption. It is crucial to use the most power efficient sensing design possible as these systems are designed to be compact and remain powered on continuously checking for entry and exit alerts.

Operation of Door and Window Sensors

Traditional door and window contact sensors are used to alert when someone enters or leaves a building or when an entry point is left open. Sensors are an essential part of security systems, making sure for peace of mind. The terms door and window sensors are interchangeable as they are both classified as contact sensors. Typically, one dimensional magnetic switches are used to recognize the alterations in magnetic field when a door or window is opened or closed; however, one concern with this implementation is that these sensors only indicate one of two states. The sensed 1D magnetic flux density is either above or below the device's trip points. As a result, it is possible to tamper with the system to think that the door or window is closed by using a strong enough external magnet to cause the switch's trip point to be triggered despite the fact that the door or window can be opened.

Also, linear 3D Hall-effect sensors can be used to sense the exact magnetic flux density in all three directions, which allows the system to differentiate the magnetic signature of a door or window actually being closed from a tamper magnet being used in an attempt to fool the security system. 3D Hall-effect sensors can be used for open and close detection in space-constrained applications that require variations in mounting such as the position shown in Figure 1. The mounting position shown in Figure 1 has the door frame raised from the door which creates a change in distance in both the x and y axes.

Figure 1 Typical Door Mounted Sensor with Raised Frame

Hall-effect Innovations in Door and Window Applications

The TMAG3001 is one of Texas Instruments three-axis linear Hall-effect sensors with a dedicated interrupt (INT) pin that acts as a system interrupt during low power wake-up and sleep mode. This device also features a configurable low power switch mode for magnetic or angle measurements. The TMAG3001 can be used in contact sensors for open and close detection as well as tamper detection. The ultra-small WCSP package offers flexible mechanical placement in space constrained applications like contact sensors.

Additionally, linear 3D Hall-effect sensors can be used in resistance to tampering. The TMAG3001 features advanced field detection capabilities and can detect magnetic fields in the x, y, and z planes, allowing the system to detect when a sensor is being tampered with. Once the device detects tampering, the system can alert the user. Figure 2 shows a testing setup of a Hall-effect sensor mounted on a sliding glass entry door. In this test, a magnet is brought close to the door from the opposite side until the LED goes off, indicating the sensor has detected a change in the magnetic field due the tamper magnet. As shown below, the tamper magnet in this test was able to trip the Hall-effect sensor through the door.

Figure 2 Tamper Test Setup

Wake-up and Sleep Mode

To optimize power consumption, the TMAG3001 features a wake-up and sleep mode in which the device can be configured to go to sleep and wake up at a specific interval to measure the x, y, z directions, or temperature data. The device asserts an interrupt signal once the magnetic threshold cross is detected and exits wake-up and sleep mode and goes to the wait state for a period of time. In the wait state, the last measured data will be stored in the corresponding result registers. The wake and sleep time period can be configured using predefined intervals. If the microcontroller does not respond to the interrupt in the wait state, then the device continues to be in wake-up and sleep mode. If the interrupt condition is not met, the device will continue to be in the wake-up and sleep mode to wake up and measure data at the specified interval. Figure 3 below shows an example where the controller responds to the interrupt during the wait state and places the device back in wake up and sleep mode. The wake-up and sleep mode conserves energy during idle periods thus making the TMAG3001 ideal for low-power applications such as contact sensors.

Figure 3 TMAG3001 Interrupt Through SCL

Significance of Wake on Change

Similarly, a vital feature that the TMAG3001 offers is wake on change, which in this mode, the device monitors either one of the magnetic axes or the angle output for a change and wakes the system by providing an interrupt. In order to save power for battery applications, such as contact sensors, the TMAG3001 remains in wake on change mode until an interrupt is detected. This device can be configured to provide a wake on change response in standby, continuous or wake and sleep mode, when an interrupt response is obtained. The new sensor measurement is used as the reference threshold for the successive measurement. Figure 4 shows the device response where the device is responsive to the x-axis magnetic field and Figure 5 shows the device response where the device is responsive to the angle measurements. Traditionally, door and window sensors continuously consume power to monitor change, but the TMAG3001 eliminates the potential energy inefficiencies that this can present by enabling relative magnetic limit checks for both magnetic field and angle measurements.

Figure 4 TMAG3001 Wake on Change with Magnetic Axes Measurements

Figure 5 TMAG3001 Wake on Change with Angle Measurements